linux-kernel - Re: [Patch v4] mm/vmscan.c: remove cpu online notification for now

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Date:   Wed, 19 Feb 2020 12:08:10 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Wei Yang <richardw.yang@...ux.intel.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        rientjes@...gle.com, mhocko@...nel.org
Subject: Re: [Patch v4] mm/vmscan.c: remove cpu online notification for now

On Wed, 19 Feb 2020 06:44:22 +0800 Wei Yang <richardw.yang@...ux.intel.com> wrote:

> kswapd kernel thread starts either with a CPU affinity set to the full
> cpu mask of its target node or without any affinity at all if the node
> is CPUless. There is a cpu hotplug callback (kswapd_cpu_online) that
> implements an elaborate way to update this mask when a cpu is onlined.
> 
> It is not really clear whether there is any actual benefit from this
> scheme. Completely CPU-less NUMA nodes rarely gain a new CPU during
> runtime.

This is the case across all platforms, all architectures, all users for
the next N years?  I'm surprised that we know this with sufficient
confidence.  Can you explain how you came to make this assertion?

> Drop the code for that reason. If there is a real usecase then
> we can resurrect and simplify the code.